Image formation in, for example, electrophotography, electrostatic recording and electrostatic printing is generally performed in accordance with a series of steps: forming a latent electrostatic image on a photoconductor; developing the latent electrostatic image with a developer to form a toner image; transferring the toner image onto a recording medium such as paper; and fixing the transferred image on the recording medium.
The developer is mainly classified into a one-component developer containing only a magnetic or non-magnetic toner and a two-component developer containing a toner and a carrier.
The toner used in the developer has been required to have low-temperature fixability and heat resistant storage stability. It has been known to use polyester as a binder resin of the toner.
However, in order to achieve high-speed processing and energy saving of image forming apparatuses, the fixing time at a fixing step has been shortened and the heating temperature with a fixing unit has been lowered. As a result, it becomes difficult to maintain sufficient fixing strength.
PTL 1 discloses a toner including: toner base particles each containing a binder resin and wax; and an external additive, wherein the binder resin contains a crystalline polyester. Here, the calorie of an area surrounded by an endothermic curve of this toner determined with a differential scanning calorimeter and by a straight line connecting the top of an endothermic peak appearing at the lowest temperature among endothermic peaks attributed to the binder resin with the top of an endothermic peak attributed to the wax having the lowest melting point among the waxes is 0.1 J/g to 10.0 J/g. Also, the toner has an average circularity of 0.940 to 0.980, and the amount of particles having a particle diameter of smaller than 3 μm is equal to or less than 5% by number.
PTL 2 discloses a method for producing an electrostatic image developing toner including a step of storing an intermediate or final product of a toner at 45° C. to 65° C., wherein the toner contains a colorant and binder resins, and at least one of the binder resins is polyester resin (A) having crystallinity.
However, there has still been a problem that a favorable balance between low-temperature fixability and heat resistant storage stability cannot be achieved in the toner.